Messages in this thread | | | Date | Sun, 1 Sep 2013 15:16:24 -0700 | Subject | Re: [PATCH v7 1/4] spinlock: A new lockref structure for lockless update of refcount | From | Linus Torvalds <> |
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On Sun, Sep 1, 2013 at 2:23 PM, Al Viro <viro@zeniv.linux.org.uk> wrote: > > How much of that is due to br_write_lock() taken in mntput_no_expire() > for no good reason? IOW, could you try shmem.c patch I've sent yesterday > and see how much effect does it have?[1] Basically, we get it grabbed > exclusive on each final fput() of a struct file created by shmem_file_setup(), > which is _not_ a rare event. And the only reason for that is not having > shm_mnt marked long-living, even though its refcount never hits 0...
Does not seem to matter. Still 66% mntput_no_expire, 31% path_init. And that lg_local_lock() takes 5-6% of CPU, pretty much all of which is that single xadd instruction that implements the spinlock.
This is on /tmp, which is tmpfs. But I don't see how any of that could matter. "mntput()" does an unconditional call to mntput_no_expire(), and mntput_no_expire() does that br_read_lock() unconditionally too.
Note that I'm talking about that "cheap" *read* lock being expensive. It's the local one, not the global one. So it's not what Waiman saw with the global lock. This is a local per-cpu thing.
That read-lock is supposed to be very cheap - it's just a per-cpu spinlock. But it ends up being very expensive for some reason. I'm not quite sure why - I don't see any lg_global_lock() calls at all, so...
I wonder if there is some false sharing going on. But I don't see that either, this is the percpu offset map afaik:
000000000000f560 d files_lglock_lock 000000000000f564 d nr_dentry 000000000000f568 d last_ino 000000000000f56c d nr_unused 000000000000f570 d nr_inodes 000000000000f574 d vfsmount_lock_lock 000000000000f580 d bh_accounting
and I don't see anything there that would get cross-cpu accesses, so there shouldn't be any cacheline bouncing. That's the whole point of percpu variables, after all.
Odd. What am I missing?
Linus
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